In the mouse Fr98 model of retroviral brain disease, the cytokine TNFalpha was found to be required for maximal disease induction. Pathogenesis in this model is mediated by viral envelope sequences and two separate regions of envelope are both required. However, the TNFalpha appeared to act through only one of these two envelope regions. This work appears to allow a preliminary separation of several mechanisms involved in this complex brain disease, which is quite similar to that induced by HIV in humans. Study of HIV infection has shown that the spread of viral infection in T lymphocytes is mainly influenced by the envelope V3 region, whereas in macrophages viral spread in influenced by the V1, V2 and V3 hypervariable regions of envelope protein. The effect of the V1and V2 regions on spread in macrophages was found to be due to an influence on viral entry at the time of infection. By using HeLa cells expressing various levels of the two HIV receptor molecules, CD4 and CCR5, the viral strain-specific effect of V1 and V2 appeared to depend on the level of CD4 expression on the target cells. In cells with high CD4 expression, V1 and V2 sequence variation was not important, but in cells with low CD4 expression, variable viral V1 and V2 sequences resulted in either high or low viral entry. This interaction between CD4 level and viral V1 and V2 sequences appeared to account for the differences seen in spread of HIV strains in macrophage cultures. This effect may also have a role on the viral strain specificity of efficient macrophage infection in vivo in the brain and other organs.